78 T. H. MORGAN. 



These eggs gastrulated and produced embryos. Sections 

 through several of the intermediate stages show that, after the 



t> *-* 



removal from the machine, the top of the egg did not regain its 

 former roundness, Fig. 15, but remained flattened. In a few 

 cases a small space was found near the top of the egg beneath 

 the ectoderm that may represent a part of the blastoccel, Fig. 16. 

 A later stage is shown in Fig. 17. The blastopore is closing, the 

 archenteron is present, but no indications of the blastoccel can be 

 found. A lump of ectoderm lies near the anterior end of the 

 embryo. It is an almost constant feature of these embryos, and 

 owes its origin to the injury of the roof of the blastoccel. Its 

 location does not necessarily mean that the normal embryo ex- 

 tends to the top of the egg, because extensive movements of the 

 cap of small cells must take place during the time of gastrula- 

 tion. Nevertheless it is true that this lump of cells originated 

 near the top of the egg, and had been carried downwards towards 

 the anterior end of the embryo. In fact the material of which it 

 is composed may represent the ectoderm of the anterior end that 

 had been carried upwards by the action of the centrifugal force. 



The results show that the blastoccel is not essential for the 

 formation of the frog embryo, since the process of gastrulation 

 may take place in its absence. This does not mean that the 

 blastociel may not be made use of in ordinary development ; in 

 fact it is made use of, since the yolk mass is thrown into it, but 

 the result does mean that the blastoccel is not essential for de- 

 velopment. Two methods of interpreting the blastoccel have 

 been followed by embryologists. The commonly accepted 

 method is to " explain " it by assigning to it a purpose. Its pur- 

 pose is to make a space into which the yolk cells can be thrown 

 at the time of gastrulation. It would seem from this point of 

 view that the blastoccel is not only a useful, but also an essential 

 organ in development. The results show, however, that this is 

 not the case. 



The other method of interpretation is that of the school of de- 

 velopmental mechanics which has tried to account for the formation 

 of the blastoccel as the results of some such mechanical process 

 as infiltration, and have assigned it to the function of producing 

 an osmotic pressure on the walls of the blastula. Rhumbler 



